Such monoliths may be formed of a brittle fireproof ceramic material such as aluminum oxide, silicon dioxide, magnesium oxide, zirconia, cordierite, silicon carbide and the like. These ceramic materials provide a skeleton type of structure with a plurality of tiny flow channels. Small shockloads are sufficient to crack or crush the monolith. Due to this brittleness problem which exists when using this type of catalytic device in connection with motor vehicles in which the ceramic monolith is located in a housing connected to the exhaust gas system, much effort has been expended in developing means for support of the monolith in its housing so that the monolith would be immune to or unaffected by shockloads. Representative of these efforts are discussed:
U.S. Pat. No. 3,798,006 discloses securement of a monolith type catalyst element in its housing by a differentially hardened fibrous lining. The monolith is supported by a felted layer or sleeve of ceramic fibers which are compressed between the monolith and a shell.
U.S. Pat. No. 3,876,384 discloses a monolithic catalyst carrier body which is resiliently mounted in a reactor casing by surrounding the monolith with a protective jacket which includes highly heat-resistant steel reinforcing means embedded in ceramic fiber and binder means.
U.S. Pat. No. 3,891,396 discloses an elastic holder for monolithic catalyst bodies. The holder consists of a metallic corrugated tube which simultaneously forms the outer wall of the exhaust conduit and which is provided with a mechanical bias which safely holds the monolithic catalyst body and presses it against an end bearing.
U.S. Pat. No. 3,916,057 discloses a process for mounting monolithic catalyst support elements which utilizes an intumescent sheet material containing vermiculite or other expandable mica. The intumescent sheet material functions as a resilient mounting material by expansion in situ. The thermal stability and resilience of the sheet after exfoliation compensate for the difference in thermal expansion of the metal canister and the monolith and absorbs mechanical vibrations transmitted to the fragile monolith or forces which would otherwise be imposed on the monolith due to irregularities in the metallic or ceramic surfaces. The sheet material is preferably made by conventional paper making techniques, although coating or extruding on a sheet of Kraft paper, polyethylene terephthalate fiber, or glass mat or fabric is disclosed. But, the patent notes that the principal disadvantages of coating a ceramic directly is controlling the thickness of the dry coating.
U.S. Pat. No. 4,048,363 discloses a laminated intumescent mounting mat using an offset of the adhesively joined layers for use in wrapping a ceramic catalytic monolith. The adhesive on the offset is covered with a release layer which is removed after wrapping to join the ends together. After heating, expansion of the intumescent material in the mat secures the monolith in its housing or covering.
U.S. Pat. No. 4,142,864 discloses mounting of a catalytic ceramic monolith by positioning a resilient, flexible ceramic fiber mat or blanket in the space between the catalytic monolith and the inner surface of the casing. This blanket is compressed upon installation of annular plug members which are inserted at each end of the ceramic monolith between it and the casing. The plugs may be formed of solid metal, wire mesh or hollow metal.
U.S. Pat. Nos. 4,239,733 and 4,256,700 disclose a catalyst coated ceramic monolith supported in a sheet metal housing by both a wire mesh sleeve and an intumescent sleeve which are positioned adjacent each other in non-overlapping fashion.
U.S. Pat. No. 4,269,807 discloses a resilient mounting for a ceramic catalytic monolith in which the monolith is surrounded with a blanket of knitted wire mesh which is partially compressed throughout its length. Overlying the knitted wire mesh is a band of high-temperature intumescent material containing ceramic fiber as a viscous caulking or paste within the matrix of the metal mesh. In one of the constructions disclosed the ceramic monolith is coated with ceramic fibers followed by surrounding it with a blanket of knitted wire mesh.
U.S. Pat. No. 4,305,992 discloses flexible intumescent sheet materials containing unexpanded ammonium ion-exchanged vermiculite flakes and suitable for use in mounting automotive catalytic converter monoliths.
U.S. Pat. No. 4,328,187 discloses an elastic holder for axial suspension of a ceramic catalytic monolith within a housing. The monolith is surrounded with a layer of heat-resistant mineral fiber material, over which lies a jacket or sleeve of good heat-insulating mineral material, and a layer made from a highly-elastic material such as foam, asbestos or glass fiber fleece, or from a metallic wire mesh. The layers provide a cushion which serves as a damping element extending within the housing over the entire length of the monolith and elastically suspending the monolith together with its ceramic fiber wrapping and sleeve against the walls of the housing.
U.S. Pat. No. 4,335,077 discloses support of a ceramic catalytic monolith by means of elastically deformable damping rings or envelopes, where the monolith is surrounded by a protective jacket of heat-resistant cement or putty reinforced with ceramic fibers or metal in the form of a wire mesh or the like. The protective jacket is enveloped around its circumference by a soft mineral fiber layer which is compressed between the housing wall and the protective jacket.
U.S. Pat. No. 4,353,872 discloses support of a ceramic catalytic monolith within its casing by means of a gas seal member formed of heat-resistant and expandable sheet material, for example, vermiculite, quartz or asbestos, which envelopes a portion of the monolith, including a separate layer of generally cylindrically knitted wire or resilient support disposed between the monolith and its casing to dampen external forces applied to the monolith.
U.S. Pat. No. 4,425,304 discloses a catalytic converter in which ceramic catalytic monoliths are supported by an elastic pad of expanded metal or steel mesh fabrics or a knitted web of ceramic fibers at their ends and are wrapped with respective cushioning layers of expanded metal or any other known flame-retardant, corrosion-resistant cushioning material.
U.S. Pat. No. 4,432,943 discloses an elastic suspension for a monolithic catalyst body in which the annular space between the housing and the catalyst body is filled with heat-resistant mineral fiber material which serves to prevent bypass of exhaust gas and as thermal insulation, and a construction where the monolith is surrounded by a mineral fiber layer and a rigid sleeve of heat-resistant metal positioned over the mineral fiber layer. The annular space between the sleeve and the housing may be filled with ceramic fiber.
In spite of the large variety of support materials available, a typical passenger automobile catalytic converter which utilizes a ceramic monolith will be supported by intumescent sheet material like that described in U.S. Pat. No. 3,916,057 or 4,305,992, having, e.g., a nominal thickness of 0.195 inch and a nominal density of 40 pcf. This material is bent to conform to the monolith and compressed during installation of the ceramic monolith into its metallic shell in which it may have a nominal thickness of 0.130 inch and a nominal density of about 60 pounds per cubic foot (pcf). To withstand the higher operating temperatures often encountered in the operating cycle of a higher gross vehicle weight (GVW) vehicle such as a truck, the overall nominal thickness of the compressed installed intumescent layer may be increased to about 0.24 inch and the nominal density may be increased to about 65-70 pounds per cubic foot as installed.
As intumescent sheet materials are bent around the monolith, a tensile stress is exerted on the outer most intumescent layer, if there are several layers, or the outer surface if there is a single layer, which can result in tearing of the surface or flaking of the vermiculite in the intumescent layer. When this happens, the sheet materials may not be useful and closure problems can result if the flakes find their way to the flanges which are used to close the outer metal casing. As can be appreciated, the thicker intumescent materials can aggravate the situation, as can the fact that the sheet materials are put under tension.
Thus, there is a need for intumescent sheet materials which minimize installation problems while providing a satisfactory mount for fragile structures, such as monolithic catalytic converters.